Yes, they do perform admirably against many opponents, but tend to fall quickly against a prepared opponent. Heavy Bolters, Autocannons, Pulse Rifles and Bolters too often spell doom for a unit of Warriors. There are, as I see it, three remedies:

1: Keep them out of range. Only an option if they have guns, preferably a Venom Cannon and Deathspitters. They won't make a dent in a dug-in Marine unit, though.

2: Keep them out of sight. This is often the best way of ensuring that they are effective. Just sneak up and try get as few turns of incoming fire as possible. This trick works best if the whole army is "revealed" at the same time, which means that there are too many threatening targets for the enemy to kill. More often than not, the Warriors are seen as a lesser threat than, say, a Carnifex or Hive Tyrant, or even better, two Carnifexes and a Hive Tyrant!

3: Get more of them. That's right. Strenght in numbers. If you have a lot, some of them will arrive to do the job. Keeping in mind that they can't be autokilled or routed, they are quite resilient agaist the correct foe (Predator with all Lascannons, Defiler, Dreadnought with Lascannon, Devastator Squad with single-shot weaponry, etc.)

If you see my deployment on this picture (from turn 2, I believe), I have tried a combination of all three tactics. The only units killable to my opponent are the two Carnifexes, gaunts and the Hive Tyrant.

The Warriors on the right are out of range and will try to stay that way until the Death Guard in the left building are occupied fighting Genestealers, the ones on the top left are facing an opponent unsuited to kill enough of them (a Predator with three Lascannons). The Warriors with guns will try to take out the Rhino and Defiler, and the winged Warriors will fly from cover to cover and counter charge the Terminators hiding behind the big towers.